The present invention relates to an improved process for the production of monomeric and polymeric poly(O-alkylurethanes) of the diphenyl methane series by the reaction of the corresponding amines with dialkyl carbonates in the presence of a catalyst and isolation of the products in a very pure form. The invention also relates to the use of the products for the production of the corresponding isocyanates.
The production of urethanes by reaction of amines with organic carbonates is known. Various methods are described in the literature for working up the reaction mixtures and isolating the products to obtain a certain degree of purity.
Monourethanes may be purified and isolated, for example, by distillation (DE 3,035,354, EP 0,323,514, EP 0,391,473). However, this method is not applicable to monomeric and polymeric poly(O-alkylurethanes) of the diphenyl methane series because these urethanes cannot be distilled without decomposition.
Recrystallization is another method of purifying the urethanes formed by the reaction of amines with carbonates. According to DE 2,160,111 (believed to correspond to U.S. Pat. No. 3,763,217), diphenyl urea formed as a secondary product is filtered off after the reaction of aniline with dimethyl carbonate, the filtrate is concentrated, and the residue is recrystallized from hexane to obtain N-phenyl-O-methylurethane. Even after recrystallization, however, the end product of this process still contains approximately 20% impurities in the form of N-alkylated anilines which are formed as secondary products during the reaction.
According to DE 3,035,354, 4,4'-bis'(butoxycarbonylamino)-diphenyl methane can be isolated after the reaction of 4,4'-diaminodiphenyl methane with excess dibutyl carbonate by taking up the reaction mixture in dichloromethane, filtering off the ureas, concentrating the filtrate, and recrystallizing the residue from a mixture of toluene and ligroin. The disadvantages of this process lie in the poor yield of diurethane (only 17% of the theoretical), and in the formation of large quantities of oligo- and polyureas which have to be converted into the diurethane in an additional process step. In addition, the mixture of dichloromethane and excess dibutyl carbonate which is formed while concentrating the filtrate has to be separated off by distillation.
U.S. Pat. No. 4,550,188 describes a similar process for isolating 4,4'-bis(ethoxycarbonylamino)diphenyl methane which is formed during the reaction of 4,4'-diaminodiphenyl methane with excess diethyl carbonate. The reaction solution is first mixed with water and the resulting mixture extracted with dichloromethane. The organic phase is dried over sodium sulfate and subsequently concentrated. The disadvantage of this process lies in the elaborate water-based working up phase using dichloromethane as the extractant. Before the excess diethyl carbonate is recycled, it has to be separated from the extractant by distillation. In addition, the end product of the process contains aminourethane and ureas (7.4%) and, also, small quantities of diamine (0.2%) as impurities in addition to 92.4% diurethane.
Accordingly, the problem addressed by the present invention was to provide a process for the production of monomeric and polymeric poly(O-alkylurethanes) of the diphenyl methane series which would make it possible to produce and isolate the products in a highly pure form by simple and efficient working up without any of the above described disadvantages of known processes. This problem has been solved by the process described below.